Important Questions for CBSE Class 9 Science Chapter 11 Work, Power and Energy

1-MARK QUESTIONS 

Question.1 Does work done depend upon the velocity of the body. [SAII-2014] 

Answer. No. 

Question.2 State the law of conservation of energy. [SAII-2014] 

Answer. It states that energy can neither be created nor destroyed. It can only change its form. 

Question.3 In a tug-of-war one team gives way to the other. What work is being done and by whom ? [SAII-2014] Answer. The winning team does work. The work is equal to the product of the resultant force and the displacement undergone by the losing team. 

Question.4 What will cause greater change in kinetic energy of a body? Changing its mass or changing its velocity ? Answer. Changing its velocity. 

Question.5 List two essential conditions for work to be done. [SAII-2010] 

Answer. (i) A force must act and (ii) There should be displacement in the body. 

Question.6 When is 1 joule of work said to be done ? Answer. When a force of 1 newton acting on a body displaces it in its own direction. 

Question.7 What is the SI unit of work done and power ? Answer. Joule and Watt. 

Question.8 What is power? What is its SI unit ? 

Answer. It is defined as the rate of doing work. Its unit is watt. 

Question.9 Find the energy in kWh consumed in 10 hours by a machine of power 500 W. [SAII-2011] 

Answer. W = P x t = 500 x 10 = 5000 Wh – 5 kWh. 

Question.10 When is work said to be done against the force of gravity ? 

Answer. When a body lifted the work is done against the force of gravity. 

Question.11 Write an expression for the work done in lifting a body of mass ‘m’ through a vertical height ‘h’. [SAll-2012] 

Answer. Work done W = mgh, where g is acceleration due to gravity. 

Question.12 When a book is lifted from a table, against which force work is done ? 

Answer. Work is done against the force of gravity. 

Question.13 Will work be done by a man who pushes a wall ? Answer. No. 

Question.14 What is the work done when the force acting on the body and the displacement produced in the body are at right angles to each other ?

Answer. Zero. 

Question.15 Is it possible that some force is acting on a body but still the work done is zero ? 

Answer. Yes, when force acts at an angle of 90° with the displacement. 

Question.16 What is the work done on a body moving in a circular path ? 

Answer. Zero, because force and displacement are perpendicular to each other. 

Question.17 Does every change in energy of the body involve work ? 

Answer. Yes. 

Question.18 What is the work done in the situation shown below ? 

Answer. Zero. 

Question.19 A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case ? 

Answer. Given, displacement = 8 m, Force = 7N Now, Work done = Force x Displacement = 7 x 8 = 56 J 

Question.20 When do we say that work is done ? 

Answer. Work is said to be done when a force causes displacement of an object in the direction of applied force.

 

Question.21 Write an expression for the work done when a force is acting on an object in the direction of its displacement. 

Answer. Work done = Force x Displacement 

Question.22 A pair of bullocks exert a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field ? 

Answer. Work done = Force x Displacement = 140 x 15 = 2100 J 

Question.23 What is the kinetic energy of an object? Answer. The energy possessed by a body by virtue of its motion is called kinetic energy. 

Question.24 Write an expression for the kinetic energy of an object. 

Answer. The expression is KE = 1/2 mv2 , where ‘m’ is the mass and V is the velocity of the body. 

Question.25 Define 1 watt of power. 

Answer. When a work of 1 joule is done in 1s, the power is said to be one watt. 

Question.26 A lamp consumes 1000 J of electrical energy in 10 s. What is its power ? 

Answer. Given, W = 1000 J, t = 10 s, R = ? Using p = W/t = 1000/10 = 100 W 

Question.27 Define average power. 

Answer. When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power. 

Question.28 Define energy. 

Answer. Energy is the ability of a body to do work. It is also defined as the capacity to do work. 

Question.29 A body performs no work. Does it imply that the body possesses no energy ? 

Answer. When a body does not perform any work, it never implies that the body has no energy. The body may have energy but still does not perform any work, e.g., a book placed on a table has potential energy but is not performing any work. 

Question.30 What is the SI unit of energy? 

Answer. The SI unit of energy is joule. 

Question.31 Does a body at rest possess any kinetic energy ? Answer. No. 

Question.32 What will happen to the kinetic energy of a body if its mass is doubled ? 

Answer. Its kinetic energy will be doubled. 

Question.33 What will happen to the kinetic energy of a body if its velocity is halved ? [SAII-2012] 

Answer. The kinetic energy of the body will become one-fourth. 

Question.34 By how much will the speed of a body, of fixed mass, increase if its kinetic energy becomes four times its initial kinetic energy ? 

Answer. The speed is doubled. 

Question.35 Can a body possess energy even if it is not in motion ? 

Answer. Yes, it can possess potential energy.

Question.36 Define potential energy. 

Answer. It is defined as the energy possessed by a body by virtue of its position or change in shape. 

Question.37 Name the energy possessed by a stretched rubber band lying on the table. 

Answer. Potential energy.

Question.38 Give the SI unit of potential energy. 

Answer. The SI unit of potential energy is.joule. 

Question.39 What do you mean by trans- formation of energy ? 

Answer. It is the change of energy from one form of energy into another form of energy. 

Question.40 Can energy be destroyed? Can energy be created ? Answer. No.

Question.41 A cell converts one form of energy into another. Name the two forms. 

Answer. It converts chemical energy into electrical energy.

 

Question.42 Name one unit of power bigger than watt. Answer. A unit bigger than watt is kilowatt. 

Question.43 When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy ? [SAll-2010] 

Answer. A stretched bow possesses potential energy on account of a change in its shape. To shoot an arrow; the bow is released. The potential energy of the bow is converted into the kinetic energy of the arrow. 

Question.44 Name at least three commonly used units of energy. 

Answer. (i) Joule (ii) Erg (iii) Kilowatt hour. 

Question.45 Name the practical unit of power in engineering. 

Answer. Horsepower. 

Question.46 Name at least six forms of energy. 

Answer. (i) Chemical energy (ii) Heat energy (iii) Light energy (iv) Electrical energy (v) Sound energy (vi)Solar energy 

Question.47 How many watt are there in 1 horse – power ? Answer. 746 watt. 

Question.48 What is horsepower ? 

Answer. It is a unit of power. 

Question.49 A light and a heavy body have equal kinetic energy. Which one is moving fast ? [SAII-2011] 

Answer. The lighter body is moving fast.